The invention relates to a method of recording information onto an optically recordable information carrier having at least two superposed optically recordable layers, comprising a first step in which information is recorded on a first one of the optically recordable layers by means of laser light to which the information carrier is exposed from a first side, followed by a second step in which information is recorded on a second one of the optically recordable layers, which second layer differs from the first layer, by means of laser light to which the information carrier is also exposed from the first side.
The invention further relates to a recording apparatus for the recording of information on an optically recordable information carrier having at least two superposed optically recordable layers, comprising a device adapted to record information on a first one of the optically recordable layers by means of laser light to which the information carrier is exposed from a first side, and to record information on a second one of the optically recordable layers, which second layer differs from the first layer, by means of laser light to which the information carrier is also exposed from the first side.
Optically recordable information carriers are generally known and are used in recording apparatuses which record data on the information carrier by means of a laser beam. The laser beam is focused onto a recording layer in the information carrier. In the case of an adequate laser beam intensity the optical properties of the recording layer at the location of the focal spot will change, as a result of which a mark is produced in the recording layer. By varying the laser beam intensity a pattern of marks can be formed in the recording layer. The recorded pattern contains the data to be recorded in coded form. An example of such an optically recordable information carrier is the CD-R (Compact Disc Recordable).
In order to extend the storage capacity of optically recordable information carriers information carriers have been introduced which comprise a plurality of superposed recording layers. Examples of such multi-layer optically recordable information carriers are described in U.S. Pat. No. 5,761,188 and U.S. Pat. No. 5,202,875. Each recording layer in a multi-layer optically recordable information carrier can be inscribed separately by focusing the laser beam onto the relevant recording layer. The recording apparatuses use a high numerical aperture (NA). Owing to this high numerical aperture the diameter of the laser beam at the location of the recording layers situated between the source of the laser beam (laser light source) and the recording layer to be inscribed (hereinafter referred to as the intermediate layers) is comparatively large. As a result of this, the intensity of the laser beam at the location of the intermediate layers will be inadequate intensity to produce marks on these layers, whereas producing marks on the layer to be inscribed is possible. Also at the location of each of the recording layers having a distance between the respective layer and the laser light source which is larger than the distance between the recording layer to be inscribed and the laser light source the intensity of the laser beam is inadequate to produce marks in these layers owing to the comparatively large diameter of the beam.
Although the intermediate layers cannot be inscribed, they have on influence on the laser beam. A part of the laser beam will be reflected, diffused and absorbed by the intermediate layers. The remainder of the laser beam, quantified by the transmission coefficient, will be transmitted by the intermediate layers. The magnitude of the transmitted part depends on the optical properties of the intermediate layers. However, the optical properties of the intermediate layers change when these layers are inscribed. The intensity of the laser beam should be so high that in all cases each recording layer in the multi-layer optically recordable information carrier can be inscribed.
It is an object of the invention to provide a method of writing information onto an optically recordable information carrier, which allows the sequence in which the recording layers are inscribed to be selected in an optimum manner.
A method in accordance with the invention is characterized in that the method comprises a first preparatory step in which the changes of the effective transmission properties of the optically recordable layers before the recording of information on the layers with respect to the effective transmission properties of the optically recordable layers after the recording of information on the layers is determined, followed by a second preparatory step in which the sequence of recording of the first and the second optically recordable layer is determined. If the changes of the effective transmission properties of the optically recordable layers are known a priori they can be communicated to the method, for example by a user or a recording apparatus. The sequence in which the optically recordable layers are inscribed can be determined in an optimum manner, for example so as to minimize the laser beam intensity required for inscribing these layers. This results inter alia in the heat generation during recording onto the optically recordable information carrier not being unduly large and in the possibility of using a comparatively simple and cheap laser light source.
A variant of the method in accordance with the invention is characterized in that the first preparatory step comprises the reading of information about the changes of the effective transmission properties of the optically recordable layers from an area on the optically recordable information carrier, which area contains information about the physical properties of the optically recordable information carrier. If the optically recordable information carrier has an area which contains information about the physical properties of the optically recordable information carrier, such as for example a lead-in area, the method can automatically read this information and derive from this information the changes of the effective transmission properties of the optically recordable layers. The area containing information about the physical properties of the optically recordable information carrier may be provided during the manufacture of the optically recordable information carrier.
A special variant of the method in accordance with the invention is characterized in that the first preparatory step comprises a first measurement step in which the effective transmission properties of the optically recordable layers before the recording of information on the layers is measured, followed by a second measurement step in which the effective transmission properties of the optically recordable layers after the recording of information on the layers is measured, followed by a comparison step in which the measured effective transmission properties of the optically recordable layers before the recording of information on the layers is compared with measured effective transmission properties of the optically recordable layers after the recording of information on the layers. If no information is available about the change of the effective transmission properties of the optically recordable layers during recording onto these layers, this change can be measured in a plurality of measurement steps. For the write operations necessary to carry out these measurement steps it is possible to reserve for example a portion of the optically recordable information carrier.
A variant of the method in accordance with the invention is characterized in that the first and the second optically recordable layer are inscribed successively, starting with the optically recordable layer situated farther from the laser light source and ending with the optically recordable layer situated nearer the laser light source if the effective transmission properties of the optically recordable layers after the recording of information on the layers have decreased with respect to the effective transmission properties of the optically recordable layers before the recording of information on the layers. An advantage of writing onto the optically recordable layers in the sequence as defined for the present variant of the method is that the maximum laser beam intensity that is required corresponds to the intensity required for inscribing the optically recordable layer which is farthest from the laser light source while the intermediate layers have not been inscribed and are consequently comparatively transparent. During the recording onto the optically recordable layers the laser beam intensity necessary for inscribing these layers will decrease according as successive layers are written.
Another variant of the method in accordance with the invention is characterized in that the first and the second optically recordable layer are inscribed successively, starting with the optically recordable layer situated nearer the laser light source and ending with the optically recordable layer situated farther from the laser light source if the effective transmission properties of the optically recordable layers after the recording of information on the layers have increased with respect to the effective transmission properties of the optically recordable layers before the recording of information on the layers. An advantage of inscribing the optically recordable layers in the sequence as defined for the present variant of the method is that the laser beam intensity necessary for inscribing the successive layers will increase to a minimal extent as the successive layers are written.
The recording apparatus in accordance with the invention is characterized in that the recording apparatus is adapted to determine the changes of the effective transmission properties of the optically recordable layers before the recording of information on the layers with respect to the effective transmission properties of the optically recordable layers after the recording of information on the layers, and the recording apparatus is adapted to determine the sequence of recording of the first and the second optically recordable layer.